1. Field of the Invention
The present invention relates to dielectric filters and dielectric duplexers, each adapted to at least two frequency bands by using a single component, and the invention also relates to communication apparatuses using the same.
2. Description of the Related Art
Regarding cellular phone systems, for example, an apparatus having a function adaptable to two cellular phone systems by using a single cellular phone terminal is known. In such an apparatus, in order to minimize the number of components, it is necessary to use a single component adaptable to two frequency bands.
For example, when a single dielectric block is used to form a filter adapted to a first frequency band and a filter adapted to a second frequency band, it is possible to form a dielectric filter adaptable to the two frequency bands by using the single dielectric block.
However, when the two frequency bands are far apart from each other, the axial lengths of resonators are significantly different between the filter adapted to the first frequency band and the filter adapted to the second frequency band. As a result, there is a problem in that a rectangular-parallelepiped dielectric block cannot be used to form a dielectric filter. Regarding the problem, for example, a stepped part is produced on the outline of the dielectric block, and a crack is thereby likely to be produced on the stepped part. In addition, since it is difficult to grasp the dielectric block itself or the completed dielectric filter, there is an obstacle caused in an automated production process.
In a filter disclosed in U.S. Pat. No. 5,731,746, a substantially rectangular-parallelepiped dielectric block is used to form two sets of resonators whose resonant frequencies are relatively widely apart from each other. In the structure of the filter, the two sets of the resonators whose axial directions are mutually perpendicular are disposed in the dielectric block so that the resonators resonate with resonant frequencies corresponding to the axial lengths of the resonators.
However, in the case of such a structure in which all the directions in which through-holes forming the resonators are formed are not parallel, a molding metal die used for forming a unit of the dielectric block has a complicated configuration. As a result, production efficiency is extremely deteriorated.
To overcome the above described problems, one preferred embodiment of the present invention provides a dielectric filter including a single dielectric member, conductor films formed on the single dielectric member and therein to dispose a plurality of xcex/2 resonators resonating at a xc2xd wavelength, both ends of each of the xcex/2 resonators being either open-circuited or short-circuited, and a plurality of xcex/4 resonators resonating at a xc2xc wavelength, one end of each of the xcex/4 resonators being short-circuited, and the other end thereof being open-circuited. In this dielectric filter, the plurality of xcex/2 resonators constitutes a first frequency band filter, and the plurality of xcex/4 resonators constitutes a second frequency band filter.
The arrangement is made in such a manner that the first frequency band filter constituted of the plurality of xcex/2 resonators and the second frequency band filter constituted of the plurality of xcex/4 resonators are formed by using the single dielectric member and the conductor films formed thereon and therein. This arrangement permits the dielectric filter to act as a filter adapted to two frequency bands.
In addition, in the dielectric filter described above, resonance frequencies of the xcex/2 resonators and the xcex/4 resonators may be set at specified values by making line impedances of open-circuited-face sides of the xcex/2 resonators and the xcex/4 resonators different from those of short-circuited-face sides thereof, and lengths of both the xcex/2 resonators and the xcex/4 resonators may be substantially equal.
In addition, in the dielectric filter of the present invention, the dielectric constant of the first frequency band filter comprised of the xcex/2 resonators may differ from the dielectric constant of the second frequency band filter comprised of the xcex/4 resonators. With this arrangement, the frequency ratio between the first filter and the second filter is not limited to a ratio of 1:2 so that the frequency ratio can be set at an arbitrary frequency ratio.
In addition, in this dielectric filter, the plurality of resonators may be formed by dielectric coaxial resonators produced by disposing inner-conductor-formed holes in parallel with each other in a dielectric block. Accordingly, structuring of the dielectric block and formation of the conductor films disposed thereon can be facilitated by disposing the inner-conductor-formed holes acting as the xcex/2 resonators and the xcex/4 resonators in parallel with each other.
Another preferred embodiment of the present invention provides a dielectric duplexer including the dielectric filter described above. For example, two sets of a transmission filter and a reception filter are disposed in a single dielectric block to form an antenna duplexer adaptable to two frequency bands.
Furthermore, according to a third aspect of the present invention, there is provided a communication apparatus including one of the dielectric filter and the dielectric duplexer described above. For example, in the communication apparatus, one of the dielectric filter and the dielectric duplexer having characteristics adapted to two frequency bands is used in a high-frequency circuit section.